During the past year, the laboratory altered its methodology for measuring BCR-ABL, converting from a "home brew" assay designed to measure the ratio of BCR-ABL to G6PDH to a commercial assay provided by Ipsogen based on the recommendations of the EAC (Europeans against Cancer) designed to measure the ratio of BCR-ABL to ABL. The laboratory committed to make this change because the ratio of BCR-ABL to ABL has become a more widely accepted standard for CML monitoring within the research and clinical community. The sensitivity and specificity of the new assay could be readily assessed and we found the new assay to be as specific and slightly more sensitive than the older homebrew. The biggest challenge in managing a transition was the need to maintain data continuity for clinical investigators. Because there is still no well defined consensus on assay conditions or pool of well stardized control reagents, the results obtained with different assays are not directly comparable. To facilitate comparisons between the old and new assays during the transition period, we performed perform parallel testing of both assays for a period of six months. Using this approach, we were able to directly measure residual disease using both assays in most patients with active disease, and for patients not seen during the transition period, we were also able to develop an equation for correlating results between assays. In practive, we found the new assay provided numerical results that were approximately 10 fold larger than those generated using the older assay. All involved physicians were notified of the change in units, and the transition to the new assay occurred without incident.